This invention relates to circuitry employing organic transistors and, in particular, organic field effect transistors (OFETs) to detect chemical odors/vapors/gases (analytes).
Many different types of OFETs are known. By way of example, FIG. 1 shows the structure of an OFET 10 having a semiconductor body region 12 with a source electrode 14 and a drain electrode 16 defining the ends of a conduction channel through the semiconductor body 12. The OFET 10 also includes an insulator layer 18 and a gate (control) electrode 20 to which a voltage may be applied to control the conductivity of the semiconductor body region (i.e., the conduction channel). The OFET of FIG. 1 is manufactured to have organic material in its semiconductor body region 12 that can absorb analytes and which, in response to the absorbed analytes, changes the conductivity characteristics of the conduction channel. As illustrated in FIG. 1, analytes (vapors/odors/gases) may flow over the OFET for a period of time. Ensuing changes in the conductivity of the OFET may be measured as shown in FIG. 1A by sensing the current (Id).
In known circuitry, the OFETs have been used as discrete devices. As shown in FIG. 1A, the source of an OFET may be connected to a first point of operating potential (e.g., VDD) and its drain may be connected via a load resistor RL to a second point of operating potential (e.g., ground potential). The gate of the OFET may be biased via resistors R1 and R2 to produce a desired operating direct current (d.c.) bias level within the source-drain (i.e., conduction) path of the OFET. The OFET may then be subjected to a flow of analytes which causes its conductivity to change. The corresponding change in conductivity of the OFET is then detectable by a circuit connected to the drain and/or the source of the OFET.
A problem with known OFETs is that their sensitivity to the analytes is relatively low. Also, known OFETs are subject to drift and threshold shift as a function of time, as shown in FIG. 2A and FIG. 2B, respectively. In FIGS. 2A and 2B, it is seen that, for a fixed bias condition, source-to-drain current (Id) of an OFET changes (e.g., decreases) as a function of time. This is the case when there is no signal input (i.e., no odor), as illustrated by waveform A of FIG. 2A and waveform portion C in FIG. 2B. This is also the case following the application of an odor to the OFET, as illustrated in waveform B of FIG. 2A and in waveform portion D in FIG. 2B. That is, for a fixed bias condition, the current through the conduction path of the OFET changes (drifts) as a function of time. OFETs may also be subjected to hysteresis and offsets. As a result of these characteristics, it is difficult to use OFETs in known discrete circuits to differentiate an input signal from background conditions and to determine or measure the full extent of the input signal.
Problems associated with the characteristics of OFETs, such as time-related drift, detract from their use as sensors and amplifiers of their sensed signals when the OFETs are used as discrete devices. Applicants recognized that OFETs should be incorporated in circuits specifically designed to overcome and/or cancel the problems associated with certain characteristics of OFETs such as their drift, threshold shift and hysteresis.
Circuits of various embodiments include at least one odor-sensitive organic transistor having a conduction channel whose conductivity changes in response to certain ambient odors and a feedback circuit coupled between an output of the organic transistor and an input of the organic transistor to generate a feedback signal which stabilizes the output signal of the odor-sensitive organic transistor for time drift.
In one embodiment, the odor-sensitive organic transistor is an organic field effect transistor (OFET) subject to drift as a function of time. The OFET is an integral part of an amplifier circuit for supplying input signals to the amplifier in response to certain odors. The amplifier may include a switch coupled between the output and the input of the amplifier circuit for providing a negative feedback that tends to cancel the effect of time drift due to the OFET. The switch is disabled to decouple the feedback loop, during times in which the circuit functions as a gas sensor.